Solid lasers represented by an Nd:YAG laser have advantages in that they are compact and easy in maintenance compared to gas lasers such as a CO2 laser and also they can transmit laser light by optical fibers. For the above reasons, various processing techniques using solid lasers have been developed and realization of high power lasers are especially expected for the use of metal welding and cutting which need high energy.
As typical methods for exciting the Nd:YAG laser, there can be LD (Laser Diode) excitation and flash lamp excitation. The LD excitation can emit light having wavelengths required for exciting a laser medium so that a laser system with high efficiency can be constructed. However, a high power laser diode is so expensive that the device cost thereof also is very expensive, with the result that lamp excitation is still mainly used in the high power laser system. A flash lamp emits light over a wide wavelength ranging from ultraviolet light to visible light and near infrared light. However, the wavelengths of light required for exciting the Nd:YAG laser are the ranges of 530 to 550 nm and 580 to 600 nm, and the light of the other wavelengths than the said ones does not contribute to the excitation thereof. Therefore, there is a problem that the laser excited by a lamp disadvantageously is low in energy efficiency.
As described above, among the light emitted from the xenon flash lamp, the ultraviolet light with a wavelength of 400 nm or less not only does not contribute to the excitation but also is absorbed into Nd:YAG crystals to turn into heat so that light emitting efficiency may be impaired or solarization may be caused. Consequently, a glass tube used for a xenon flash lamp, such as M382 produced by Shin-Etsu Quartz Products Co., Ltd. is advantageously made to block the ultraviolet light. However, even in this case, the ultraviolet light is only blocked, which does not contribute to improve energy efficiency.
As a silica glass capable of converting a light wavelength, Patent Document 1 discloses a silica glass doped with divalent copper and aluminum. However, the silica glass is handled with complication due to two kinds of dopants. Further, there had been a problem that an optimum wavelength light for exciting Nd:YAG cannot be obtained from the silica glass because the fluorescence peak wavelength thereof is 500 nm or less. In addition, Patent Document 1 is silent on bubbles in the glass, and viscosity and an OH group concentration thereof The method for producing the silica glass shown in the Examples of Patent Document 1 is a sol-gel method in which a large number of OH groups remain in the glass and it is very difficult not to generate bubbles therein.
Patent Documents 2 and 3 disclose a copper-containing glass having a fluorescence peak wavelength ranging from 500 nm to 600 nm and a controlled OH group concentration. Especially, the glass disclosed in Patent Document 3 is a wavelength converting silica glass which is preferably used as a flash lamp for exciting Nd:YAG and improved oscillation efficiency of the laser is observed by using the silica glass.
In recent years, as a result of higher power laser having been developed, a flash lamp is required to have higher power and higher durability. However, when a flash lamp produced with the glass of Patent Document 3 was used under high power, although efficiency was clearly increased at an initial stage of the operation, deterioration of oscillation efficiency of the lamp was often observed after prolonged use. In other words, even the wavelength converting silica glass described in Patent Document 3 has not been capable of meeting the laser oscillation with high power and for long time which has recently been required.
Patent Document 1: JP 5-254879 A
Patent Document 2: JP 2005-187262 A
Patent Document 3: JP 2005-272243 A